Generalist bird pollination of a winter-flowering African succulent, Aloe marlothii

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The importance of birds to A. marlothii

Features of flowers that are attributed to a bird-pollination syndrome (ornithophily) include: i) diurnal anthesis with nectar peak and flower opening in early morning; ii) large pollen-nectar distance; iii) vivid colours, bright reddish-orange flowers; iv) absence of odour and nectar guides; v) distinctly curved tubular flowers; vi) secund flower arrangement, and; vii) abundant nectar held in flower by capillarity (Faegri and Van der Pijl 1979; Smith 1991; Maclean 1990; Proctor et al. 1996). These features are nearly all represented in A. marlothii. However, it is still unclear who pollinates most aloe species.
The flower morphology of A. ferox and A. candelabrum suggests an ornithophilous syndrome (Hoffman 1988, Stokes and Yeaton 1995). Similarly, Amarlothii with its horizontal to slanting racemes (Van Wyk and Smith 2005) is suggestive of a system focussed towards bird pollination. Inflorescences are robust, being able to support large bird species such as grey go-away bird and columbids (>300 g) (Proctor et al. 1996). Generalist bird pollinators are most often attracted to flowers with low concentration (c. 8-12%) and high volume (c. 40-100 μl) whilst specialist nectarivores prefer nectar of high concentration (c. 15-25% w/w) and low volume (c. 10-30 μl) (Johnson and Nicolson 2008). The long tubular flower shape of most aloes excludes certain insect species, although honeybees Apis mellifera are able to access the nectar of some flowers (Reynolds 1969; Hoffman 1988; Human 2006, Human and Nicolson 2008). In A. castanea the open perianth makes the dilute (8- 10%) nectar accessible to bees that prefer pollen (Nicolson and Nepi 2005); whilst in other aloes with a fused perianth, constricted tubular corolla and tightly packed stamens (like A. marlothii), nectar may be more difficult for bees to access. Aloe marlothii flowers during periods of little to no rainfall, so it is likely that copious nectar production supplies birds with both water and a sugary food source (Oatley 1964). In A. ferox four of five bird visitors and five of eight insects were classed as pollinators (Hoffman 1988), and for A. divaricata in Madagascar the Souimanga sunbird Nectarinia souimanga appeared to be the most effective pollinator (Ratsirarson 1995). On Mayotte Island, the endemic A. mayottensis was primarily pollinated by a single sunbird species, Nectarinia coqureli, with occasional visits from Zosterops maderaspatana (Zosteropidae) and an unidentified hymenopteran (Pailler et al. 2002). Numerous cases have demonstrated the importance of insect visitors for plants (see Proctor et al. 1996).
In many instances a specialised pollination syndrome is indicated in which a plant taxon relies on a specific pollinator (or specialised group of pollinators), as occurs with the relationship between nemestrinid and tabanid flies and long-tubed flowers (Manning and Goldblatt 1997; Johnson and Steiner 1997). Aloe vryheidensis has been demonstrated to filter flower visitors with long bills (those perceived as nectar robbers) through increased concentrations of bitter tasting phenolic compounds in the nectar (Johnson et al. 2006). Despite obvious perceptions of a bird pollination syndrome, the contribution of different animals to pollination of A. marlothii remains to be investigated.

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CHAPTER 1. Production of copious dilute nectar in the bird-pollinated African succulent Aloe marlothii (Asphodelaceae)
Materials and Methods
Study site.
Phenology and flower development
Nectar measurements .
Statistical analyses .
Acknowledgements .
CHAPTER 2. Response of avian nectarivores to the flowering of Aloe marlothii: a nectar oasis during dry South African winters
Materials and methods
Concluding remarks
CHAPTER 3 Generalist bird pollination of a winter-flowering African succulent, Aloe
Materials and methods.
CHAPTER 4. The importance of Aloe marlothii nectar for an avian community: evidence from stable carbon isotope
CHAPTER 5 Interpreting an Aloe marlothii nectar diet shift using stable nitrogen isotopes
CHAPTER 6 Isotopic routing and its implications for the utilisation of sugars and water
in Aloe marlothii nectar by opportunistic avian nectarivores


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